Phase shifting circuits, or phase shifters, are utilized in a variety of applications and devices to shift the phase of an input signal. For example, the input signal can be a radio frequency (RF) signal or microwave signal that is utilized in a radar system, antenna array, etc. Depending on the specific system and level of complexity, the number of individual phase shifters can exceed tens of thousands.
Phase shifters can be implemented as digital or analog components. Digital phase shifters (or digital shifters) often utilize PIN diodes to select between a limited number of discrete phase shifts that can be applied to the input signal. A PIN diode can be switched on and off by changing its resistance from around 100 kΩ to less than 1Ω. This can be achieved, for example, by altering voltage bias of a PIN diode from forward bias to reverse bias direction, or vice versa. The number of discrete phase shifts is often described in terms of an exponential value based on the number of bits in the digital phase shifter (or digital shifter). For example, a two-bit digital shifter would be represented as 2n, where n is the number of bits, thus providing a total of 4 possible states. Similarly, a three-bit digital shifter would provide 8 discrete states (i.e., 23=8), a four-bit digital shifter would provide 16 discrete states (i.e., 24=16), a five-bit digital shifter would provide 32 states (i.e., 25=32), etc. A three-bit digital filter with a 360° range could, for example, implement the following discrete states (or phase shifts): 45°, 90°, 135°, 180°, 225°, 270°, 315°, and 360°.
Digital shifters, however, have several drawbacks. The resolution is limited because number states are discrete. Using the above example, phase shifts ranging from 46° to 89° are not possible. Digital shifters are also current controlled components that typically require about 10 mA per diode. In applications that require tens of thousands of 3-bit digital filters, the total current consumed at 5V can exceed 1 KW. In complex antenna configurations that require tens of thousands of digital shifters, the power requirements can result in heating and thermal interference problems. Digital shifters also have an insertion loss that increases with the number of bits and with frequency increases. In the Ka band, for example a four-bit digital shifter has an insertion loss of around −3.2 dB and a six-bit digital shifter has an insertion loss of −5.2 dB. Such losses can be intolerable for efficiency sensitive systems.
Analog phase shifters (or analog shifters) often utilize a varactor diode (or simply varactor) to provide continuous phase shifts within a particular range. Varactors operate in a reverse biased condition providing a junction capacitance that varies based on applied voltage. The phase shift can be changed continuously with control input, thereby providing unlimited resolution with monotonic performance. Unlike digital shifters, an analog shifter could provide continuous phase shifts from 45° to 90° with a high level of precision (e.g., 45.00001°, 45.0001°, 45.001°, etc.), based on the applied voltage increments. Varactors are also voltage controlled components with minimal (or negligible) power requirements.
There are several drawbacks associated with analog shifters. Varactors have a breakdown voltage and their variable junction capacitance is limited by the breakdown voltage. This results in limited range of phases that can be covered (up to about 100°). Thus, sufficient phase shifts cannot be provided to cover the full 360° range without excessive signal loss.
Based on the foregoing, there is a need for an approach for providing continuous phase shifts over the full 360° range with minimal insertion loss and minimal power requirements.